28" F/4.4 Ritchey-Chretién Cassegrain refiguring

Yes, that's right - it's not a typo, it's a Cassegrain with a system focal ratio of F/4.4.
The primary is a perforated 28" F/1.1 mirror, and is full thickness!
The secondary is 8" in diameter and disturbingly close (in terms of distance) to the primary mirror (about 22") when compared to the other Cassegrains I have made or worked on.

Here is a photo of the telescope, sans optics, in its horseshoe-type mounting.  It is nearly as wide as it is long!

Assembled telescope and mount
Assembled 28" telescope, sans optics

So how did this project come about?

Well, a good friend received an email from a frustrated telescope owner (who asks to remain anonymous) and he forwarded that email to me.  Little did I know that this would lead to a nearly one-year odyssey into some very extreme optics.

Many emails and telephone calls ensued following our initial messages.  I learned more of the details of the troubled optical system with every message.  Tests were conducted and results relayed, ideas were discussed, and it was decided that I would attempt to fix the optics.  In July of 2006 after a long drive, the optical tube and optics were delivered to my shop by the owner.

Over the coming months I designed and constructed test equipment to allow the primary to first be tested, then refigured.  Testing revealed that the optics were not terrible, but they were certainly not up to optical quality in the visual wavelengths and likely were used originally for infrared imaging.  The goal for the telescope after refiguring was to provide resonably good CCD images in the center of the field, with the addition of a corrector anticipated in the future to help correct for the serious field curvature and astigmatism that shows up off-axis in a fast RC.

The primary was tested by creating a tester that formed a point source from laser light, and allowed observation of the return image.  I'm not going to say any more than that for now because I don't want to say any more - I just don't feel like explaining.  It was not inexpensive to build and is quite heavy, but it allows accurate zonal measurements to be taken from zones anywhere on the mirror.  Using it as a point source for testing a mirror that is nearly spherical, it is a powerful and revealing test for checking for astigmatism after polishing.  I now use it regularly to test fast optics and to ensure that my optics are a VERY good figure of revolution.  (No, it's not an interferometer.)  No matter what anyone says, Foucault testing is simply NOT adequate for detecting figure-of-revolution errors on moderate to fast focal ratio mirrors.

Over the course of a couple of months I learned to take repeatable readings with this tester, and I determined the primary mirror to be a bit overcorrected for the system prescription I was aiming for.  However, to determine the figure, I first had to write my own test analysis software because other software would not handle the number of zones I wished to test, nor did I trust it with such a fast mirror.  I had been looking for a good excuse to write my own code and now I had it!  The code was written as a Matlab script and works fine for my purposes.  (No, I have not converted it to C and I have no immediate plans to.)  A photo of me working on the primary (all done by hand) is shown here.

Me at work
Me refiguring the 28" F/1.1 primary

As the primary was a decent figure of revolution, I removed the high areas and reduced the error to what I measured to be around 1/4 wave on the glass.  At this point I declared victory on the primary.  After getting the primary close enough, I planned to tune the overall figure of the system by figuring the secondary.  Therefore, I was most concerned with smoothing the surface of the primary mirror, as it seemed to have some zones in it.  I used techniques that both smoothed and adjusted the figure.  (Based on the final results, I believe I smoothed the primary quite a bit.)

With the primary done and uncoated, I could reassemble the telescope and test the whole system....... after I built more test equipment.

So how does one test a 28" F/4.7 RC system with two uncoated mirrors?  Two words - collimated light.  How do we collimate light?  We use a Newtonian telescope in reverse.  A focused laser is used to create a very bright spot that is a little smaller than the airy disk for the telescope.  This spot is placed at the focal plan and voila, if the diverging cone of light has sufficient angular spread to illuminate all of the paraboloidal primary, a beam of well collimated light the diameter of the primary emerges from the system.

So why not use a large flat?  I am capable of making one (I even have a 24" blank), but a coated flat used to test the scope in autocollimation would have the light bouncing off four uncoated surfaces, and even with a laser the return beam was sure to be dim and alignment of the system somewhat tricky.

I bought a couple of 24"x2" mirror blanks, one flat and one generated with an F/4.75 curve.  The flat blank was packed safely away to make a flat from in the future, and I completed the 24" F/4.75 mirror in January of 2007.  I made it as good as I possibly could, and checked it carefully for astigmatism with my new tester.

The collimator structure was built in the coming months, and the system was assembled after the mirror came back from coating.  I used a HeNe laser rated at 1-2mW.  After collimating the system as best I could by moving the pinhole source around (rather than tilting the primary), I put my 4.25" F/4.5 Newtonian in front of the collimator, placed a ronchi grating in the eyepiece, and had a peek at the results.  I smiled as I observed straight ronchi bands, shown below, knowing that testing complete telescopes up to 24" in aperture and beyond was now easily manageable in my shop.

First ronchi image taken with collimator
First test of collimator system:  85 LPI Ronchigram of my homemade
4.25" F/4.5 Newtonian using collimated laser light

The collimator also serves as a convenient way to help in the setup of a new telescope, as it is basically an artificial star in my shop, no matter what the weather is!  One can CAREFULLY use an eyepiece to observe a "star" image.  This is done with a welding filter to reduce the brightness for telescopes with aluminized optics.  This allows me to make sure that all my eyepieces come to focus without taking the telescope outside.

The next step was to reassemble the 28" RC, place it in front of the collimator, collimate the 28" (easier said than done), and then observe the Ronchi lines through the 28" telescope for an eccentric 24" diameter portion of the aperture.  For comparison, here are some Ronci test images taken with the assembled telescope with a real star as the source.  Significant correction error is present, along with zoniness.  It is followed by actual images of stars, which never really came to focus.  (I did not create these mosaics.)

Ronchi, star is source - before refiguring
Ronchi testing of 28" RC system using 133LPI grating and a real star as a source (BEFORE refiguring)

Star images
Defocused images of a real star taken with 28" RC system (before refiguring)

The owner had requested a change in back focus, so the primary-secondary spacing had been altered slightly to place the focal plane where he had requested.  Surprisingly, my first tests of the reassembled system showed reasonably good correction, but a number of zones were observed.

28" test before secondary work
85 LPI Ronchigram of 28" F/4.4 system after primary work, but before secondary refiguring

Now let's get something straight here - this telescope is never going to be perfect, not did I ever suffer from that delusion.  It simply needed to be significantly better than it was before I worked on it.  That in itself would be a good outcome for such an extreme telescope.  So, I plunged into the work on the secondary and emerged a month later, tired but quite a bit wiser.  After determining that some distortion was present in the ronchi lines at the top and bottom of the test images due to gravity distorting the collimator mirror and the poorly supported primary mirror (which was never intended to be supported entirely on edge) and also due to the method used to retain the primary (don't ask), I determined that the most reasonable test was with the ronchi lines aligned horizontally.  The image below shows the final result of refiguring.  Note that a 133-line-per-inch (LPI) grating has been used for this image rather than the 85 LPI used in the previous images, so the image is more sensitive to error.

Final test
133 LPI Ronchigram of finished 28" F/4.4 system after primary and secondary refiguring

While it may look a bit rough for a "normal" visual telescope, this is really good for such a crazy system and MASSIVELY improved over the system as it was used before I worked on it.  I am proud of the outcome, but at the same time I hope to not see something this fast for a long, long time!

The telescope was picked up by its owner this July (2007), and he was especially pleased to use an eyepiece and view the "star" image from my collimator as he went in and out of focus - apparently he had never seen it focus properly, but now it was there, right before his eyes.  At that moment I felt relief, satisfaction, and a bit of joy.  Another successful project completed, soon to be coated and exposed to starlight for the first time.  What a great hobby/career.

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